The present invention relates to an electrical signal mixing device and particularly to a device commonly termed a "mixer" in the field of audio reproduction and recording.
Such devices are used to route incoming input signals from a plurality of signal sources to one of two output channels or one or more selected sub-group channels selected from a plurality thereof. From the sub-group channels the signals may then be further mixed, again in selected combinations, into two final output channels constituting the left and right recording channels for stereophonic reproduction. For other more specialised applications there may be a greater number of output channels.
Each input and sub-group channel includes means for modifying the parameters and characteristics of the incoming signal, and the precise combination of routing connections determines the nature of the ultimate recording. Because the mixer may be used in different circumstances, even if permanently located in a single recording studio, the number of input channels required and their interconnection with the sub-group and output channels varies substantially from time to time. This is further complicated if the mixer is used, for example, in the generation of output signals during concerts, since the different venues may place different constraints on the siting and number of the signal sources, constituted for example by the microphones or other transducers from which the input signals are generated for the input channels so that changes may have to be made even for the performance of the same musical item when played at different places. Because of these factors it is often the case that a mixer of greater capacity has to be acquired in order to be able to cope with anticipated future developments. Initially, however, substantial parts of the mixer may not be used and if the anticipated developments do not materialise, or if requirements develop in a different way from that anticipated, the mixer may have to be exchanged for one having a different channel arrangement.
In conventional mixers, for the routing of input channels to the sub-group and/or to the output channels mechanical switching units are employed. This means that each input channel must have an associated switch for each of the potential output or sub-group channel connections which may be made. In a mixer having thirty two input channels, twelve sub-group channels (even if grouped as six stereo pairs) and two output channels, this would require 224 double pole switches. This large number of switches constitutes a not inconsiderable part of the cost of the mixer and, moreover, because of their mechanical nature, also represent a potential source of breakdown.